I think ὅσος would have been the common choice, but due to the subordinate statement
referring to secondary tense, we have opt. and the indefinite ὁπόσος. The dative case
appears to be required by the verb inside the comparative(?)* clause.
If the verb inside the comparative clause is the same as that of the leading one,
then it is commonly omitted (Smyth 2464
* I use the term "comparative" even though I'm not sure it is the case here. The construction
alone seems to fit the bill, sans comparatives or superlatives.
To me, your translation is spot on.