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One cannot check alignment often enough because as weight is added the spars tend to sag so they need constant checking. We also used intermediate supports under the rear spar to keep the spars level.  We installed the centre ribs at midpoint, 1/4 points and jumped around so that the ribs helped to keep the spacing as the ribs were installed.  A simple check for spar level is to take two blocks of wood & hack saw a depth so that both are the same height. Place one at each end on the spar web & run a string line through the sawcuts. Tie one end and hang a weight on the other end to pull the line tight. Set the line at a height that it will pass through the bottom of the nose rib lightening holes. This is where a metric ruler helps as it is finer graduations. Adjust the cut & set the string at both ends the exact same height above the spare web. For example: 45mm. Now with the string line tight you can take the ruler and check that the measurement is the same between the spar web and the string. Less than 45 it needs to go down & more than 45 is sagging and needs to come up. We left this set up on all the time for quick checks and definitely when returning to start another session. The alignment will change & this is a quick way to keep the spars flat & parallel.  Measure from the top of the rear spar to the top of front spar as this needs to be at 32" all the way as well. Top to top is the same distance as center to center and is an easy read. Add a bead to string lines used to align jig holes as it is easier to see the clearances around the bead to centre the jig hole as a string through a 3/16" hole is a coarse read w/o a bead. We borrowed an automatic survey level to shoot in the spar ends to set the wing jig frame supports holding the spars which may be overkill but it is very accurate start.  A quality builders level be very accurate as well and may be rented.  If the wing is perfectly aligned on assembly then you can stand at the end of the wing and look through each of row of jig holes & it will appear as one hole through all the ribs to the other end of the wing. ..  Glenn BH727