This is easy. What has been found best is to calculate the amount of T3 in the meds that you are already taking and to take that amount of pure T3 in place of it.
This should mean that you feel very little difference for the first day as the T4 levels sink too slowly to notice overnight and you have kept up the levels of T3 you were already taking.
After 5 to 7 days you will start too feel hypo with sinking temperature and pulse and then be ready for your first T3 increase.
The Desiccated Thyroid meds all contain 9mcg of T3 per grain and you can work in round numbers using 25mcg T3 tablets divided into halves or quarters. Suggested "round number" starting places are:-
0.5 Grain - 6.25
1 grain = 12.5
1.5 grain = 12.5
2 grain = 25
3 grain = 25
4 grain = 37.5
5 grain = 50
6 grain = 50
You will need to spread this out through the day and will need to increase after a week or so. This is covered on the main dosing page.