Oil pump chain tension

[Rallyace 3]

Does anyone have a definitive answer on how much deflection there should be on the oil pump chain. I am fitting a 26 tooth sprocket and 52 link chain to retain good oil pressure at tick over in my 1900 8 valve rally car.  Without a spacer there is, what I consider to be excessive slack in the chain. I realise that I'm looking at it in a state that it won't actually operate in. I'm assuming that when the chan and sprockets are "wet", the film of oil will take up a small amount of slack and that the block will grow a small amount due to thermal expansion when it's hot. How are you guys measuring it. Given how long the engine has been around and how many have been tuned and rebuilt, there must be a deflection, or range that is known to work and be safe/reliable.  

[SRDT 182]

You need a 1.1mm spacer for the 26 tooth sprocket so obviously there is too much slack if you dont fit any.

The XU9JA has a 0.7mm spacer for a 22 tooth sprocket and 50 link chain. That is if it's an engine made after 04/1987.