On January 1, 1944, the Bomber Command order of battle lists 1,436 bombers. This does not include OTUs, other training aircraft, and liaison aircraft, but includes the special-duties units. Early that year, the total strength of Bomber Command was about 155,000 personnel, and I didn't write "men" because it includes the WAAF, as well as all other ground personnel.
Assuming the –70% reduction is linear and brought forth as a firm decision throughout the war, this would mean that at this point Bomber Command would have some 430 bombers, and 46,500 personnel.
Bomber Command Operations
The area bombing doctrine cannot make sense with this cap on the force. Bomber Command has to work on targeting, in order to make that 30% of its OTL bomb tonnage really count. This will make for an even slower start of the campaign than in OTL, but may yield some unexpected dividends. Bomber command might cooperate more closely with some USAAF ideas, for instance following up at night the Schweinfurt raids, using the fires as beacons; and thus concentrate on German production bottlenecks. Another example: with this small a force, power stations would probably be targeted, which would have been a smart idea even in OTL. The oil campaign would see a more determined commitment, etc.
There is a down side to this all, of course. No extensive destruction of German cities. No extensive loss of industrial manpower. No resources and money spent on repairing and rebuilding; which means the better targeted, more focused damage described above will be easier to clean up (see below). Plus, many maintain that German morale was not significantly affected by the city bombing. That might be true or not, depending on who you ask, but it is perfectly possible that without widespread destruction of cities the German morale would be _better_, regardless of the fact that maybe the factories would lack ball bearings, or the military vehicles would like even more fuel than in OTL, or the electric power would go off at random.
Other Uses of the Manpower
The British underwent a growing manpower shortage from 1943 on. With 108,500 more personnel, many more things could be done. Not all of this personnel would be suitable for infantry service (remember those WAAFs are included); but a sizable part of that shortage was in the industry (remember the "soldiers" working in the coal mines at the end of the war). Turning no more than 50,000 of this surplus into infantrymen, the Italian campaign or Operation Goodwood might easily be a tad faster. Note Bomber Command creamed mostly from the higher end of the manpower spectrum. Such a small bomber force probably means many more good officers and NCOs for the Army.
It is however quite likely that a fair share of this surplus would still go to the RAF; only, not to Bomber Command. One wonders if, once there is no longer such a pressing need for pure fighters, the British develop a much more robust fighter-bomber, tactical support force. Which again might well be a good idea. One supposes there will be more for the naval side of things, embarked Seafires, Coastal Command units helping out against the U-Boote, that kind of things. More artillery observation aircraft would also be good towards the end of the war.
Other Uses of the Industrial Potential
Of course the aircraft industries develop way less; they can still build more fighters, maritime patrol aircraft, etc. Even so, there's more manpower and resources available here, too.
And the Germans...
Apart from the points above, which depend on the likely choices by Bomber Command, there are the choices the Germans themselves will take. Much less night fighters, searchlights, AA batteries – deployed in Germany, that is. They might well turn up on the Eastern Front or on the Westwall (in the case of the fighters, in the form of daylight units, of course). Most of that reinforced concrete goes on the beaches instead of in those huge shelters.
The very large civil-defense organization, rescue teams, repair and labor outfits etc. will be largely available to concentrate on the more focused damage the British bombers will do, which will cut the advantage.
Life will be much harder for the USAAF, almost unbearably so after the introduction of long-range escorts, and totally so before that. In fact if those German night-fighter units are daylight-fighter units, the daylight skies of Germany might get terribly crowded with black crosses (depending on whether they are redeployed there from the East). Which might in turn nip a series of Schweinfurt-like attacks in the bud, including their night follow-ups by the smaller Bomber Command.
And the 60% Figure...?
Where does this come from? Thank you.