1986 Pontiac Fiero adjustment

Trans in question is 5 spd. Muncie getrag. The repair manual I have states that the select cable is non adjustable however the new cables I bought are both adjustable (shift and select), which leaves me with the question what length should the select cable be? The manuals says to lock the shifter in 3rd gear and adjust shift cable at the trans. Which I assume they mean at its furtherest travel point, being 1 3 5 position. Having done this I seem to be in the ball park with it but it doesn, t feel right or shift smoothly. Seems to want to pop out of gear in 2 4 R positions. The car is still on jack stands so I have not tried to drive it yet and seems wise to not do so. Do you suppose I am the shift cable is overtraveling in 1 3 5 and not fully engaging 2 4 R?