The secondary use of television spectrum (TV whitespace) by low-power radio devices is now a reality in the UK and the USA. The UK regulator Ofcom has released rules for computation of maximum allowed transmit power. These rules, however, allow some leeway for different algorithms for internal steps, and one would like to exploit this leeway to the advantage of secondary users, while still protecting the primary user to the required degree. We describe here two techniques implemented in a current BT software product, which additionally allows a compute-on-demand mode of operation, in distinction to so-called TV whitespace databases which merely look up pre-computed tables.