It may be a good thing to check the splines on the clutch basket for condition before going any further by pulling the basket. Don't forget they are an opposite thread.
At this point it may save you some time and you know the condition then. Usually the lower gears have a bit more wear than the uppers.
It is normal, the mainshaft has about 30 degrees of lash in it in 1st gear. I have the primary off my bike right now and the top off the transmission, I put it in 1st gear and get the same lash as you do. Put it back together and ride. -Dan