Spoke Size / Spoke Length Database - What size spokes do I need?

I'm going to build fixed gear rear using a FRONT Shimano Deore M756 disc hub with a bolt-on cog from VeloSolo.

I've bought the hubs and some rigida DP 18 rims. Now I'm browsing for spokes.

I want the rear wheel to be non-dished and I've worked out chainline etc.

Now the question is, in this Shimano spec sheet, centre to flange distance is documented. Does that sound right. Can I just pop that information, along with the 100mm OLN, 61mm flange diameter and 581mm ERD into a spoke calculator and trust the result I get? I got 277. Anyone with a similar setup who can back that up as accurate?

http://bike.shimano.com/publish/content/global_cycle/en/us/index/tech_support/tech_tips.download.-Par50rparsys-0034-downloadFile.html/07%29%20Hub%20Specs.pdf

Thanks very much,
M

A few other questions:

1. Is OLN irrelevant if the wheel isn't dished?
   
2. I actually got 277.8, shall I round down to 277 or up to 278?