PAR formula to change air reading to water?

[polyppal]

I have our local clubs 'budget' PAR meter, which measures with an Apogee meter on a voltmeter (and you calculate it x5 to get par value). Seems to work pretty well in water, but I am trying to calculate what the estimated PAR is in my new tank without water. Its under 2x36" Reefbrite XHOs and 2x36" T5 ATIs

Is there a formula or way to estimate the difference for a water filled tank from measurements on a dry tank?

 

[rtparty]

I think the immersion effect is 1.08?

 

[polyppal]

I think the immersion effect is 1.08?

id imagine that it declines more rapidly the further the depth too, right?