Sizing question for all the experts here. I'm looking at some new frames and I want to make sure I get the correct stack for my fit. My current bike is a 58cm Jamis with a 58cm stack height and a 180mm HT length. I have 3cm of spacers from the top of the head tube to the bottom of the stem. Do I just add this 3cm to to the stack height to get my actual number or is there more involved? Doesn't seem like there should be more than that but I'm not 100%.

This is the bike and you can see the spacers. I really don't want more drop than this, I'm just not flexible enough.