How much straight is straight enough

[Wladimir Kummer de Paula]

Yesterday I have built myself a new building surface. I´ve used a 18mmg 
thick MDF that I reinforced with two aluminum extrusions lenghtwise. I 
think is pretty straight, but I still noticed a very small (say 
0.2-0.5mm) gap due to bowing at the middle.
Is that acceptable?
How straight is straight enough?

Thanks

Wladimir
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