I have found that expression in this passage of Alvarez’s De Institutione Grammatica:
Illinc [in rationem viamque tradendo qua Latinam Linguam discas] omnia quae potius impedimento quam emolumento esse possunt, omnino, quoad ejus fieri poterit, removenda sunt. (De Nominum Declinatione, 3.)
Lewis and Short says it means “as far as possible“; why is that “ejus” used? “Quoad fieri poterit“ would literally be “as far as it can be made“, which makes sense, but that “ejus“ does not seem to refer to anything previously mentioned.
The sense is: as much as this [precept, suggestion, recommendation] is possible - or in this case: to avoid [it] as much as possible. ejus = partitive genitive. “as much of this command/dissuasion” as will be possible. Not an uncommon part of scholastic Latin classroom vocab.
I guess the fossilized scholastic use of the phrase derives from the initial definition of the orator’s duty in the much earlier Rhetorica ad Herennium (once thought to be Ciceronian), which runs (1.2) oratoris officium est de iis rebus posse dicere quae res ad usum civilem moribus et legibus constitutae sunt, cum adsensione auditorum quoad eius fieri poterit. There I take eius as a feminine partitive genitive referring to assensione auditorum. Evidently in scholastic Latin it became part of the fixed phrase and lost its original meaning.