As a rule the silver Age omnibuses follow three mansterwork formula with a few notable exceptions
So lets explore how things will look if Omnibus 3 is 4 masterworks rather than three for the future omnibuses

Three Masterwork version

Amazing spider-Man Omnibus 3
 68 -99

Amazing spider-Man Omnibus 4

100- 131

Amazing spider-Man Omnibus 5

132 - 166 approx

Amazing spider-Man Omnibus 6

167 - 200 aprrox

Amazing spider-Man Omnibus 7

201 - 223 Approx

Amazing spider-man omnibus 8 will of course be Roger Sterns upcoming one

4 Masterwork version of omnibus 3

Amazing spider-Man Omnibus 3

68 - 109

Amazing spider-Man Omnibus 4

110 - 142

Amazing Spider-Man Omnibus 5

143 - 180 approx

Amazing Spider-Man Omnibus 6

181 - 209

Amazing spider-man omnibus 7

210 - halfway into Roger Sterns run

Looking at it like this i think the 3 masterwork rule actually breaks quite nicely and almost perfectly ties in with Roger Stern Omnibus although granted that last omnibus is a bit on the small side