Battening - getting them flat and true

[Dreadnaught]

I'd love some guidance from the great people here.

Its about battens for external cladding and how to get them flat and true.

 

I've got most of them up and and all is going well. But inevitably I've found a couple of bulges in the racking board which I think I should flatten.

Is the best approach to pack the battens with plastic packers if they are a bit low. And if they are a bit high use a cheap electric planer, which I can buy from Screwfix, to take a few mm off them?

 

I'm thinking to aim for a tolerance of 2mm at most. Is that enough?

Am I along the right lines?