Thank you Rob / NickFL,

The tape's cross-sectional dimensions are 1.3 x 0.85 cm.

Yesterday, I came up with a possible solution. From a prevoius experiment where I coiled the heating tape on a PVC pipe, I have the cylindrical area covered with 5 m of the heating tape. I can now associate this quatity (heat delivered per m2) with the heating tape linear length I have in the new experiment (20 m). I'll test this I let you know the outcomes.