Well, usually people figure time ratio's of their area, how much day and how much night. it's 12/12 here and i have about 4 hours of shade on mine total, 8 of direct light. I would say at least 6 hours of direct light and if it's not so direct but light, then at least 8 hours i'd say.
What i'd do is take a whole day, get up when the sun does and write down the time that the plants actually get light light (any at all, like when the sun comes up), then go there and time just light where they arent direct but it's getting some sun, and write it down. keep checking them every 20-30 mins and see when the regular light ends and when it gets direct sunlight and then write down the regular light end, then the direct sun light beginning. then go check when it changes again to something else and write down the direct sun light end and the new phase beginning, just do that till it's night time and you'll have a time chart of when your plant gets direct light, normal light, and no light. then you'll figure your ratio and how much direct light etc...
About the mylar, you should try to go to home depot or some place like that. Yes it would help because the light would reflect, but only if the light comes from the opposite direction that you have the mylar but might be a bit obvious because you can see through fence cracks so if someone looked over they'd see a big shiny mylar on the fence and wonder what it was, that is if your hypothetical situation isn't so "hypothetical" lol.
anyways good luck bro