This is an estimate, a guess, based on a fairly comprehensive reading of the studies and what I see on forums.
A really simple, low risk formula would be just use 2/3 the tirz dose of reta, so 15mg tirz to 10 mg reta. If you want to be extra careful, then 1/2 the dose. Anything more conservative than this seems pretty unnecessary, there is no guarantee you wont get more side effects from that swapover as everybody's reactions to different glps are a bit different, but it is not likely. Reta is a bit more likely to increase heart rate more than tirz, so if you have heart disease, either don't use reta as it is not studied in that context yet or be very slow and careful with dose increases.
Fairly low risk and saves months of messing around upping doses super slowly. Once you see what side effects you get from the first dose or 2, you should get a better idea of how it will work for you and what to do with dose increases from there.