Here's 2 ways to do it. With the quarter round removed for either way, select the straightest board that you cna find in your pile of adequately acclimated flooring. The wood should have been in or very close to the room where it's going to be installed for a month--3 weeks absolute minimum--to allow it to acclimate to the rooms humidity. Set the board along one wall in the direction that the floor will run. Grab your old high school compass and set that for the widest width gap that there is between the wall and the board. Then mark the board full length with the compass and remove the wood that is on the edge you marked. You can remove it with a block plane, jig saw, belt sander or any other method that works and doesn't split the wood badly.
The next method starts out the same but does use a slightly different method of measuring the gap. Set the first board as before and use the ruler part of a combination square to set the width from the wall. Keep moving along with the blade as you mark along the wall and mark the other edge of the ruler with a pencil. Then remove the wood as before but you will be removing more of it this way. It may also be easier to do with your available tools. You can use this square or your framing square to mark around doorways, pipes and similar intrusions pretty easily also.
I know that this might be difficult to understand and a picture would be a LOT easier but if you play around a little with the wood and tools, it will probably be easier yto understand.
I you will need more than one board length for the room, make up an "assembly" with a couple of clamps and the 3 straightest boards so you can do the wall all at one time.