so if in the first start you have the font size and the label size that are correct and after changing you only need to change the label height so the font will fit inside
then change the same % as you change to the font size
like if font size is 18 and label height is 40 then if you change font size to 20 then this is (100/18 * 20 = 111.11 so 11.11% bigger) now add the same to your label height
40*1.11 = 44.44
(just a thought...
, not sure it will do it but it worth a try..)