How much sand

I have a new
triton II TR100 sand filter that the instructions call for 600 pounds of sand
to achieve a free board of 11 1/4 inchs
The dealer install used 350 pounds of sand to achieve a free board of
17 inchs. When I questioned this the response was this is the way we have
always done it with good results. Its a good filter you will be happy with
the results.
Who is right. dealer or instructions?

That's an easy one. Make him put in the rest of the sand. Your filter is operating probably around 60-70% of it's capability.