In my calculations example, Ive used a fixed label size of 50mm.
That includes the margin you have mentioned.
When printing from the software, you select a label length. Ive opted for 50 for the testing Ive been doing. With that setting, I can do a preview. It text is too long it gets truncated in the preview and print. I will produce various length label options in Ideas Genie Pro (e.g. 40, 50, 60 and 70).
If number of characters is small, the label length is fixed as described above, so the user may wish to use a shorter label option and vice-versa.
When you print from the keyboard on the PT2730, and use chain output, you can produce something similar to your example. The label length is shown in the printers monitor as you type characters. If too long you can change the font size, or split text over 2 or more lines (depending on the height of your label stock).
One line of 10pt text looks good for 9mm tape. Bold makes it even better.