Ron, getting a first course straight is quite a task. I have snapped a line, use a laser level, done geometrical formulas, and there is actually no true way to ensure this will be right. there are no reference points in a home to rely upon to be true.
There is one way to get close is to use the 3 2 5 rule. People use this when laying line for a deck. It finds perpendiculars or right angles. this could be a test when you think you have found a straight line.
Ron, remember, the base moulding will be your safegard against a small mistake. You know that right???
I am sure you have thought of this, but I will say it just in case. Do not get within 1/4 or 1/2 inch of the wall on either wall. This will allow for movement. If you do butt it up, the floor will surely buckle up.
Base molding will cover up the void and 1/4 round or shoe molding comes in many sizes.