Hi 'grizzly. There is no measured height at which a screen needs wipers, it depends on whether the driver has to look through it to see the road ahead. The construction and use regulations 1986 say the following -
Every vehicle fitted with a windscreen must have one or more efficient automatic windscreen wipers, unless the driver can obtain an adequate view of the road to the front without looking through the windscreen.
So, if your eyeline is above the top of the screen you don't need wipers. There has been at least one Sammio Spyder that an MOT tester with little man syndrome failed because he couldn't see over the screen, but the simple expedient of removing the screen before the retest solved that one.
Good luck.
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