Trouble understanding dependent fields

I'm attempting to make it possible for a user to order multiple t-shirts with their event registration. Per posts I found on the forum, I've set up a field for the t-shirt sizes and prices, then created dependent/custom fields for the quantity for each size. Then, in each size field, I put in a custom formula of, for example, ([SM_T_SIZE]-1)*[TSHIRT] and associated the size field with just one of the sizes in the T-shirt field (I'm not sure why the -1 is required, but it seemed to make it calculate the pricing correctly if you select just one size).

My issue, as you can see from the attached screenshot, is that if you select more than 1 size from the checkboxes and then change the value to more than 1, it doesn't calculate the total correctly. The sizes selected in the screenshot are $20 each, but the calculated total is as if I selected 4 shirts total.

I hope I'm describing this well enough for someone to tell me what I'm doing wrong. Tuan, if you want to look at this on the site, I've sent you credentials for dev.uwpiaa.org in that support ticket we were working on earlier.

Thanks!