The mainsheet is the most important adjustment on the Lido and should be constantly adjusted on all points of sail to keep pace with the changing conditions. A general setting for upwind sailing is to maintain a top batten position that is either parallel or just hooking to weather of the boom. Be aware that in light air the weight of the boom will prevent the top batten from opening. When this occurs you should ease the main out to get the top batten parallel to the centerline of the boat. As the wind builds you will gradually increase mainsheet tension to maintain proper top batten alignment.
Many people find that a telltale placed on the top batten is helpful for trimming the main.
Though this does not provide a consistent guide, the main does seem to set well with this telltale on the verge of stalling. In a lull or when you need to increase speed you should ease the sheet and in heavy air or when going fast you should tighten the sheet. Like the jib, the secret to proper mainsail trim is the correct combination of both power and pointing. Offwind - When reaching ease the main out as far as it can go without luffing. When sailing downwind, the main must go out until the boom just touches the leeward shroud.