You are correct that SEHID cannot be greater than the amount you actually pay out of pocket.
If you don't mind the dinosaur approach . . . I used to calculate this manually when I was too cheap to buy tax software. It's an iterative process as you know because income affects PTC which then affects SEHID which then affects income. I found that only 2 or three rounds were needed to get within rounding error.
Try your scenarios, find the one that works best for you, and note what the PTC and SEHID should be; then enter your info into Turbo Tax and double check that the PTC and SEHID calculated like you expected. If everything is correct except the SEHID, there is a way to overwrite it in Forms Mode but note that any entries in forms mode invalidate the accuracy guarantee. At that point you might want to call or chat with an expect to see if they can't find a way to finagle it w/o you having to do an overwrite.
