I might humbly suggest 5.5%
So you're suggesting a 0% premium on unsecured debt with a nation, which if it loses the war, might be unable to repay unsecured creditors. Could I interest you in the purchase of a slightly used bridge?...
In reality, the Entente might only suffer a modest increase in finance costs initially, but interest rates are likely to steadily rise with the balance of unsecured debt - ultimately there is a limited appetite for unsecured debt, unless an appropriate risk premium is on the table . The level of British procurement from the USA would initially need to be scaled back compared to existing levels. This is a total contrast to OTL, where the entry of the US provided almost limitless credit and resulted in a flood of war materials to the Entente cause - over and above the relatively modest military contribution of the USA. The immediate shortfall of US material would ITTL almost immediately and increasingly impair the Entente's combat performance compared to OTL.
Further, if the Entente were to continue to attract purchases of unsecured debt at reasonable rates, it would need to demonstrate an Entente victory (and ultimate repayment of unsecured Entente debt) remained viable. Conversely, potential investors in Entente debt would be digging for information upon which to make their investment decisions. So not only would the Russian February revolution create a crisis of confidence, but otherwise suppressed problems like, French manpower crisis, French morale crisis, French fuel shortages, British naval fuel shortages, British Army fuel crisis, British manpower shortages, British tanker shortages and British naval shortages are far more likely to be publically identified and make potential investors ever more nervous.
So, the Entente access to credit (and therefore US materials) is gradually and increasingly strangled, until Russia taps out and the Entente access to unsecured credit dries up altogether.