Heating Placement Range

[hoad]

Hi guys, just finished off a new viv for my SWCP and im putting in a new globe, a 150 watt infared spot lamp but im not sure how far to put it away from the basking shelf without it being too hot or too cold, Keeping and Breeding australian pythons says that roughly 33 - 35 degrees celcius is optimal for SWCP, any ideas roughly how far to put the shelf to achieve this?

Thanks a lot, Nick.