Cannot measure technical debt?
Martin Fowler has posted that technical debt cannot be measured. It probably is impossible to establish an exact measurement method for something so complex. But a team of developers always know when their velocity is being undermined by technical debt that needs refactoring away. In general I find that talking openly with a development team provides a more useful and reliable picture of the state of affairs than any number-crunching could ever do.
So instead of aiming for a precise metric why not just ask the team how much technical debt they think has accumulated? The answers can be classified (e.g. None / Some - work mostly unaffected / Some - work often affected / Lots - work constantly affected) so that a fair decision can be made about the amount of debt to repay in the current iteration.