Argon2id key derivation rate calculation question

Just wanted to know if this was an accurate way of determining how long it would take someone with an 8GB GPU do derive 1 million keys using some assumed parameters below:

Time per derivation...........: 3.5 seconds Memory required per derivation: 128MB GPU Max Memory................: 8GB 

So, 128MB = 0.125GB. Max derivations that could be happening at any given time = 8/0.125 = 64.

Each derivation takes 3.5 seconds, so derivations per second = 64/3.5 = ~18.3

So, 1 million key derivations would take (1000000×18.3)/3600 = ~15 hours.

Does this seem accurate, or am I unknowingly making too many assumptions? Thanks.