How to know focus free setting of large format lens?

[YDegroot@xxxxxxx]

I have made my own "V-pan", using a 4x5 Graflex focusing bed, and bellows, a 
homemade box with a 6x17 rim, and removable 6x17 rollfilm backs. I have two 
lenses, both from, Osaka, (4-element Ektar/Tessar design) and can focus using 
a 6x17 groundglass: a 120mm and a 180mm. 

However, for quick shooting without wanting to focus, I want to be able to 
set the lenses at "infinity", so that I do not need to focus and can just 
increase my depth of field by using smaller f-stops.

Does anybody know at what distance I can consider these lenses focused at 
infinity?

The 90mm lens on my Linhof 617 focuses at infinity beyond its 40 feet mark, 
at about 50 or 60 feet, which is about 15 meter.

The 120 must be around 18-20 feet
The 180mm around 80 feet (twice the distance of the 90mm????)

To anyone who can helkp, thanks in advance.