Rule #1 of accessible design. If you're thinking in pixels, somebody isn't thinking!

I have my old and new 5" phones. one is 640x1024, the other is 140p. I have two tablets, one is 1440p, the other is 1080p.

The pixels per inch across all those devices is NOT consistent, nor is their default font-size or scaling. This is why NOTHING but the smallest of units like thin borders should be declared in pixels if you care at ALL about accessible design.

Use EM/REM instead. If 2.5REM isn't "right" then there's probably something WRONG with what you're doing.

2.5 REM equals:

40px for normal / VGA / 16px users.

50px for 8514 / Windows XP/earlier large / Win7/newer medium / 20px users

60px for Win7/newer Large

80px on a ten foot UI 4k media center.

Mobile devices also let the user set the system font size, which does inherit to the browser if it's configured properly.

The solution isn't some goofy calculator that STILL results in wildly different sizes across devices, it's to stop using the ignorant broken nonsensical pixel measurement!

As I'm always saying they're called EM, use 'em! If you are "designing" in pixels, you don't know the first blasted thing about design.