Distance from primary air input and floor located 'P' channel?

What is the ideal distance between the primary air input and a floor located 'P' channel in a batch box firebox?

Thanks as always for any help.

Mine is a 150 mil system and is endlessly tested. The distance between the air inlet and the start of the floor channel is just over 50 mm. I don't know what the ideal distance would be, provided there's one. But the system as it is works without a hitch, again and again.

Thanks for the quick reply Peter. I'll use your tried and tested 50mm distance.