The ideal tuning range for each drum in isolation:
8" - F—F#
10" – C#—D
12" – A#—B
13" – F#—G
14" – E (maybe F if rack tom)
15" – D—D#
16" – C—C#
18" – A#—B
I try not to go more than 1/2 step up or down from the notes suggested tuning range: too high will have more overtones with less fundamental — tape or moon gel will be needed to focus the note and stop each head’s tone from ringing separately during sustain. Too low will sound more spiky and flappy due to decreased resonance, plus will be harder to maintain tuning and will dent more easily.
Once set-up, 2" difference between drums is not enough to get intervals of 4ths between each drum while keeping each in their perfect range. Ideally drums will have 3" difference between them if you want evenly spaced intervals (e.g. 10", 13", 16" or 12", 15", 18")
Real world set examples, optimising for ideal note relationships:
3 up, 1 down:
8", 10", 12", 14/16" – G, D, A, E/D
2 up, 2 down:
10", 12", 14", 16" – D, A, E, C
10", 12", 16", 18" – D, A, D, A
2 up, 1 down:
10", 12", 14" – D, A, E
10", 12", 16" – D, A, C
10", 13", 16" – D, G, C
1 up, 2 down:
13", 16", 18" – G, D, A (16" will be overtone heavy and require lots of muffling to sound good next to the 18") or A, C, A, (but minor third between floor toms not ideal)
12", 15", 18" – B, D#, B
1 up, 1 down:
12", 16" – F#, C# or A, C
13", 16" – G, C