I find that for any DIY mix I do - no matter the flavor, I add my sweetener based on overall amount of juice I am making.
Example: If I am making 10ml's of RY4, I add 4 drops of Flavor apprentice "Sweetener" as well as 4 drops Flavor apprentice Ethyl Maltol. I have found that as a "rule of thumb", I use 2 drops sweetener (whatever sweetener you choose) to every 5ml's of E-Juice. I am a little on the sweet side as far as sweeteners go, as I hate to taste any of the "Alkaline taste" that seem to be imparted from the PG/VG.
Of course, keep in mind that anything that any of us tell you here, is all based on OUR personal taste/preference...this ain't science that's written in stone it's just a "starting point" for you to tweak from there to YOUR liking.
Me, I seem to put TPA Sweetener and Ethyl Maltol in EVERYTHING! That's me....you may not. Recently, I have also found that adding TPA "Smooth" in all of my mixes - be they fruity, Tobacco or a menthol juice really seems to give the vapor that "rounded out" feel and adds to a feeling of a "fuller" vapor closer to what cigarette smoke "felt" like...if that makes any kind of sense!
Bottom line: EXPERIMENT, EXPERIMENT, EXPERIMENT, EXPERIMENT, EXPERIMENT! Yeah, you'll waste some juice here & there and make some unbearable junk juice...so what! Just expect that to happen. Actually, PRAY for it to happen! It will help you to get closer to nirvana alot quicker when it's all said and done.
DIY ain't for the faint of heart! If you are afraid to screw up here and there, (not saying you are) then stick to buying it. I have found that you really have to use that part of your DNA that isn't afraid to say "Hey watch this". I find that this mentality, while not compatible with firearms or explosives; is an absolute necessity when it comes to DIY
Ok, that's my .02 and It didn't cost ya .03 to get it...lol
Good Luck and just go for it!