When Square Is Not Square

I am having a hard time working out my Very Hungry Caterpillar quilt top.  The reason for this, according to Himself, is because I haven't done a quilting class where they teach you the right way to do things (he is very big on "the right way to do things")...I would love to do a quilting class but it is not high on my list right now - besides, I said to Himself, who needs a quilting class when you have the Internet?  So many tutorials, finished pieces, works in progress etc., and the funny part?  Everyone has their own way of doing things...I think it's just trial and error to see which way works for me.  I love the guidance a teacher can offer but I really do dislike the restriction of being told How Things Must Be Done!

Sadly though, while I am working off the world's simplest idea for a pattern (just the panel in the centre and borders around), it really is hard for me to guestimate the amount of fabric required - and yesterday, purchasing some beautiful tone-on-tone purple, somehow I thought 50cms would be enough and it clearly isn't.  Rather than going back to buy more today, I have adjusted my pattern idea.  Either way it looks as if I will have to purchase more of some fabric, just not sure what yet - and the metre of a particular print that I did purchase to go with it, I am not happy with now and don't really want to use.  So far, I have bought five pieces of fabric for this quilt top and I don't want to use two of them, one of them I didn't buy enough of and the other I definitely don't have enough of with my adjusted plans.

I do agree with Himself when he tells me that I should just follow a pattern (any pattern!) with fabric requirements etc. included, it would be a lot easier! 

The other problem I have come across is this: the caterpillar & butterfly panel (one rectangular piece) was not cut straight on the sides.  I wanted to trim it straight so I could get ready to add the first border...each piece is on a white background with a green border but neither white "square" is actually square.  I have separated and trimmed each block from the panel (so it is now in two, roughly square, roughly the same size pieces) by measuring the same amount out from the white line against the green border, this way I have the same amount of green (more or less) all the way around.  But they aren't square squares!  How do you contend with this?  Do you iron each square into as square a shape as you can manage?  Do you pin the borders to the square following the original white line (and possibly end up with skewiff squares)?  I am unsure of how to proceed...

I am used to working with things like glass (and even so, not very well I might add).  Glass is a lot more restrictive than fabric but on the other hand, if you square up a piece of glass, it stays square, the shape doesn't warp.  I wanted something more flexible / less restrictive than glass to play with, so I have turned to fabric but I am finding it behaves in ways just as peculiar.