Metronome markings

[Bubbles]

The song Im arranging now has a tempo of Quarter note = 98.5. Should I change it to 98 or 99, since thats not even really a number and your average metronome doesnt have that speed? Im positive its that speed; I played the songs at the exact same time and 99 seemed too fast, 98 too slow, but 98.5 was the right speed

[Bubbles]

Oh, and one other thing that has almost nothing to do with the first question. When would you use a sheet with the left hand in the treble clef and the right hand in the treble clef with the 8 on top of the clef sign(I know what it is, like the 8va but idk what its called) instead of just both hands in the treble clef?

[JDMEK5]

The song Im arranging now has a tempo of Quarter note = 98.5. Should I change it to 98 or 99, since thats not even really a number and your average metronome doesnt have that speed? Im positive its that speed; I played the songs at the exact same time and 99 seemed too fast, 98 too slow, but 98.5 was the right speed

I would leave it at 98. I would make it a whole number for sure because even digital metronomes that can beat any speed can't hit fractionals.
And I would choose 98 because on non-digital (that's not the word I'm looking for) metronomes, I believe they have 98 but for sure not 99.

When would you use a sheet with the left hand in the treble clef and the right hand in the treble clef with the 8 on top of the clef sign(I know what it is, like the 8va but idk what its called) instead of just both hands in the treble clef?

Perhaps if the right hand is played (for most of the piece) so high that the only alternatives are tons of ledger lines, or a super-long 8va.
Both of which are really ugly.

Let me know if I misunderstood your questions.