You can find all the answers to your questions including spreadsheets to help you design an inclined manometer and orifice plates on this site:
Flowbench Technology
All you need to do is follow the new member post in the General section to join. You will only see the standard forums till you become a member (there is no charge for membership). I have had a lot of forum members build some pretty serious home built benches!!
I also supply sharp edge orifice plates, velocity probes, plans and coming soon a 3 sensor digital manometer that will be priced for the DIY'er but offer Professional features! These parts are being used in quite a few Professional shops across the world now. No waiting list for the parts either, they are made in my shop.