
I've tried it all and couldn't get a perfect alignment between the grid printed on my maps and the one on roll20's screen. I got close enough to look well enough, but the tokens were the real tell-tale sign that it was really off.
Playing Pathfinder, I set my squares to represent 1/2 of the normal square size (2.5 ft. rather than 5). Then set the grid to 0.5 rather than the default of 1.
Any token you put on the grid will be very small as the squares on the grid are now 1/2 the size. The solution, make all medium sized creatures (tokens) 2x2. They will snap onto the grid with finer control because there is twice the number of anchor points.
This only works if you turn the Opacity down to nothing. You and your players use the printed grid lines on the map and still have the ruler tool be accurate when figuring out distance.