[itsme]

No action required regarding text size calculation. Something has changed, but I can easily tweak the text margings to get what I want.

Regarding
" Tap the number value to the right of the slider if you want to make small adjustments (the dialog that is shown lets you press -/+). I can look into adding the same slider used in the annotation editor and text display settings though."

The +/- buttons in a follow-up dialog as in the Page Margins can imho much better be operated with the fingers. When I change sizes in the text display  settings, it happens frequently that I touch the slider instead of the + or - button and set a value to the maximum or minimum.

But:
The follow-up window of the Page Margins has a bug: 
When I change a value in the follow-up window with the +/- buttons and press OK, the follow-up window is closed AND the Page Margins window is also closed, my change is not taken over.
Expected behaviour: pressing OK in the +/- follow-up window shall pass the changed value to the Page Margins window and return to the Page Margins window that stays open. Closing Page Margins with OK shall take over all changes, closing Page Margins with Cancel shall discard all changes.
This bug existed already in MobileSheets 3.7.7 and is still there in 3.8.6 on Win10.
It works correctly in 3.8.6 on Android.