@tapio
correct, I still believe current behavior works correctly. As I wrote, there is a need for one tiny change that causes a little trouble for me. If you set 150% to magnify, the app just needs to do zoom by this 50 % (like by fingers) and then all will work as expected. Anyway, let it be for now, I'll invest more time to it and in next Beta ...
It's slarti76

Nevermind...
Tried out 3.38.4. First tried with 200% and 300%: Absolutely identical maps displayed. Then tried out 150%: No change between 100% and 150%. Then, as you hinted at, zoomed out and in again: Now it looks like it may be 150%. Tried out 200 and 300 again: Now there was a difference.
At last, tried 400%: Different to 300%, but muuuuch bigger.
Now I noticed the percentages shown in the top bar. At base resolution (no magnify), it showed "200%". Ok, probably a result of the DPI calculation, whatever. With magnify to 300%, it showed "500%" (which is 2.5x=250% magnification). At 400%, it showed "1000%" (which is 5x=500%).
No it struck me: The problem here is, I think, that my map doesn't have linear zoom levels, the highest levels are "1:50000", "1:25000", and "1:10000". I assume that when I'm in the "1:10000" level, that messes with your calculation, as you expect the lower level to have half the resolution (1:20000) and calculate the level to be used for magnification and the enlargement of tiles based on that - which has to fail.
With a Google map download done by Locus, I don't have those severe discrepancies, but there's also no difference between 250% and 300%, for example.
Also, when I zoom in more than the highest zoom level (one or two additional taps on "(+)"), I'll still get to a point where Magnify has no effect.
So in summary:
- The algorithm seems to break down when the map's zoom levels are not 1:2
- Magnifications between full 100% steps are sometimes a problem*
- Magnifying when already magnified in highest zoom level doesn't work
- Sometimes magnification is not applied directly, only after another zoom in/out
* I think it would be no loss to get rid of the 25% steps, and also all 50% steps after 200%, but at least 150% should work.
Hope this helps.
PS: Good news: Didn't have any blank screens anymore