What % of Retin-A to Use with 8% Prescribed Hydroquinone?

Hi my doctor prescribed me 8% hydroquinone, but neglected to give me some retin-A because I was probably butchering the pronunciation of its medical name, "Tretinoin".. and so I want to ask, what percentage of Retina-A should I ask him to prescribe me, and whats the strongest percentage of it available? Just to reaffirm, I've already been prescribed hydroquione but do not intend on using it till I have retina-A.

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