Because of the way penalties and estimated tax payments are calculated, if you do the conversion before May 31, you will owe 1/4 of the tax amount on April 15, June 15, Sept 15 and January 15, 2026. That means one payment is already overdue. You can mitigate that by paying 2/4th of the tax (or more) by June 15, and using the annualized method on form 2210 to show that you don't owe a penalty. If you do the conversion after June 1, you would want to pay at least 3/4th of the amount by Sept 15, and the rest by January 15.
The way around this is to have tax withheld, and make up the difference in the Roth IRA from the other funds you would have used to make the estimates. For example, you convert $5000 and have $1250 withheld. That means that only $3750 goes into the Roth. You can then send $1250 to the Roth IRA from other funds within 60 days, and tell them it is a "rollover". They don't have to know or care that it is part of the same rollover/conversion, that will be settled on your tax return. Because of the way penalties are calculated, having the tax withheld eliminates the need to use the special form to eliminate the penalty.
