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The vacuum requirements are the same for any direction, normal or reverse. I feel you should be using a vacuum of at least 20in Hg. The greater the vacuum, the better the holding power.

Maximum vacuum is about 29.9in Hg.

I have a Gast vacuum pump which is able to generate around 27 - 28in Hg depending on how well the wood seals, the porosity of the wood.

Take a lot at the site "Joe Woodworker". He has a lot of information on a DIY vacuum system.

http://www.joewoodworker.com/veneering/welcome.htm

I cannot tell you what capacity pump you need. Too many variables. What size bowl, how much piping, are you using standard copper fittings vs e.g., Swagelok, how good is the seal on the adapter for the headstock?

If the piping system and seal of the wood were perfect, then capacity would not matter, a smaller unit would just take longer to extract all the air than a larger unit.

In practice, there are always leaks. The wood itself may be porous. The more leakage, the more you want a higher capacity pump than a lower capacity.
 
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