What exactly is a unit of range for this game?

So every range in league is just a number. But for the life of me I can't figure them out. While I have through practice figured out the range all the champs I play regularly, just how big is a square unit in this game?

Using the straight mid lane for example, how far is it from inhib to inhib?

Of a secondary note..perhabs a practice tool that lays down a grid thats like 5*5 grid in the unit measures for figuring out, so from this bush I can shot to x landmark ect.