partial re-rerolls on random tables

Mucking about* with my Gamma World setting, I was thinking about a random encounter table. I wanted a specific creature to sometimes be mixed up in the encounter, about a fourth or fifth of the time.
Sure, I could just do a simple "roll again on this table and add creature X" but that would have a chance of giving the same result (and ignoring that result would result in more rolls).
I could write it as "roll a d10/d8 on a result of 1 and 2 add 1d3 creature X", but I wanted the chance of 1 creature X to appear to be higher than 3 X's. But something that I never thought of before; by rolling a d8 and d10, you can create a d80. So, here is a d100 encounter table with an incorporated d80 reroll system:

Murder City; night encounters

1-2 gravorgs
3-5 shadow sloths
6-10 skeletons
11-15 zombies
16-21 ratmen
22-26 mutated rats
27-28 quinpod invader
29-32 albino apes
33-36 octobats
37-44 bugs
45-52 ghouls
53 demonic abductor
54-59 shroommen
60-63 dabbers
64-66 felinoids
67-72 roll on the humanoid table
73 roll again and on the humanoid table
74-78 roll on the faction table
79 roll again and on the faction table
80 roll twice
81-92 roll 1d80 on this table + 1 creeper
93-98 roll 1d80 on this table + 2 creepers
99-00 roll 1d80 on this table + 3 creepers



*Lying in bed, thinking about cool mutants.