TT calculates the maximum amount I can contribute to a Solo 401(k), but not the max for a Solo Roth 401(k). Would it be the same amount? I have the latter.

Your maximum Roth 401(k) contribution for 2021 is net earnings from self-employment or $19,500 ($26,000 if over age 50 in 2021) whichever is less.  Net earnings are net profit from self-employment minus the deductible portion of self-employment taxes.

You can get TurboTax to figure this for you by selecting the Maximize box for an individual 401(k).  Whatever amount TurboTax tells you is the maximum amount that you can defer is the amount that you can instead make as a Roth contribution.  At that point, click the back button until you get back to the page where you marked the Maximize box and enter up to $19,500 in the regular Roth 401(k) elective deferrals box (but not to exceed what TurboTax told you for your maximum deferral) and the rest in the box for a catch-up contribution.  Whatever amount of the total deferrals you designate as Roth reduces by the same amount the amount of regular elective deferrals you are eligible to make.