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 - 168

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

Ok now we no that Masterwork 16 runs to Amazing spider-man 168 a bit further than i antisipated lets see how that changes things if at all

Amazing spider-man MW 17  270 pages approx
Amazing spider-man 169-180
Amazing spider-man Annual 11
Nova 12

Amazing spider-man MW 18 240 pages Approx
Amazing spider-man 181 - 193
Amazing spider-man Annual 12 cover only


Amazing spider-man MW 19 254 Pages Approx
Amazing spider-man 182 - 202
Amazing spider-man Annual 13
Spectacular spider-man Annual 1

Amazing spider-man MW 20
Amazing spider-man 203 - 212
Amazing spider-man Annual 14

Amazing Spider-man MW 21 aprox 290 pages
Amazing spider-man 213-223
Amazing spider-man Annual 15

Ok assuming this map is broadly right and again assuming the Omnibuses will match up with the Roger Stern Omnibus then two of the omnibuses need to include 4 masterworks or the last ones simply becomes an oversized Hardcover covering the Last two masterworks
so here's my prediction for how it will unfold

Amazing Spider-man Omnibus 3 Masterworks 8,9,11
Amazing Spider-man 68-99

Amazing Spider-man Omnibus 4 Masterworks 11,12 & 13
Amazing Spider-man 100 - 131

Amazing Spider-man Omnibus 5 Masterworks 14,15,16 & 17
Amazing Spider-man 132 - 180

Amazing Spider-man Omnibus 6 Masterworks 18,19,20 & 21
Amazing Spider-man 181 -223

Omnibus 3 & 4 are full of Classic Tales therefore it makes more sense to bulk out the less classic 5 & 6