It's almost impossible to predict exactly what temp you will get in the bin from the heat tape. With that being said here are some ideas.
You can place the heat tape directly under the bin and set it to run at 90 - 95 and it should warm a portion of the bin up to that temp.
For boas I have found that overhead heat works beter than belly heat. This is my preference and works best for me. I keep a hotspot of around 110F and the cool side of the enclosure around 70 (depending on room temp). I know some people think it's too hot but I've never had any get burned and they seem to thrive with it.
I've found that it's easier to give a range of temps and then let the snake or lizard sort it out. I then make adjustments based on where they spend most of their time. you need a fairly large enclosure to accomplish this, however.
As far as the reflectix, it really doesn't matter where you put it, it's just there to insulate the box and stabilize temps. I would modify the lid and use overhead heat (lightbulb fixture, dome lamp, rhp, etc..) This is assuming it's not in a rack system where this would be tough to do.
I wouldn't be too concerned if the temp is 75 and not 78. As long as you provide a gradient you will most likely be fine. Most snakes will use the hotspot to warm up then move back to their hide or wherever they feel comfortable. Don't be so concerned with exact temps, focus more on how your boa responds to the temps. If he's always on the hotspot the tank is probably too cold. If he's never on it it's probably to hot. People seem to get too hung up on numbers. Hotspot must be 92.5 degrees and humidity at 72% etc. Exact numbers really aren't that important. There good as a base to start with but that's really it.