I am not looking to be spoon fed a chart and be told to memorize it. I want to do the work myself but I am struggling with how to construct ranges and validate them. How do I tell if a hand is going to perform well against an opponents range?

For example, what am I calling and not 3! from LP against an MP raise? How do I go about checking this? Just plugging random flops into Equilab and seeing how that goes?

Or said differently

If someone is raising 20% of hands what do I want getting in there with?

There are obviously a lot of pre constructed range charts available for you to use and adapt, and the key word is adapt. Don't blindly follow these as rules, but as guidelines in which to develop your game. A lot of the building and formulating comes down to experience and is situationally relevent depending on the table, the format and your opposition.

You'll find a question like this will bring about a pool of subjectivity. One person may say that vs a CO open, in the SB we should raise KQs (for example) where I would say to flat...unless, the BB is aggressively squeezing then in which case, I would say raise. (See how that situation quickly changed...)

If I were you, I'd stick to the basics, play and play and play, and develop your own ranges based on the situations you are finding yourself. Start from scratch, look at your opens from each position and develop a range - don't let anyone tell you that you're wrong for opening 22+ UTG at 6 max for example (I've done this for years, yet I've lost count at the number of people that have told me I shouldn't be opening 22 UTG - it's my UTG range, I'm happy with it, I make it work, so there!)

Over time, fine tune your ranges, your strategies and keep learning continually.

Good luck mate.

^THIS

As for your questions on when to flat in LP against a MP raise, normally folks will flat with small to medium pocket pairs and 1 gap suited connectors.

As for 3! in LP against a MP raise, most folks save their more speculative hands for this play, i.e. suited 2 gappers, or AXs or AXo type hands, and obviously any premium hands.

I would say to open your range if the same person (in MP) is raising 20% of the time in front of you, as I think that 20% tends to be on the higher side.

One size doesn't fit all though - as was discussed in the post above me