Tables are a good lo-tech reference but are limited because they cannot account for your actual dive profile and rely on pressure groups for repetitive diving, which cannot consider all tissues so make a conservative guess as to which might be controlling. If using tables for planning, it's safest to use maximum depth. But if you're looking back for a comparison with your dive computer after you've finished diving, average depth would be a closer (but less conservative) approximation.
If you're interested, why don't you download your dives into Subsurface? You can then look at the NDLs (or deco requirement) calculated by Subsurface at any point in any dive. You can also see the effect of changing gradient factors or compare Buhlmann gf with VPM-B models. What's reported by your dive computer may also be shown (depending on what your dive computer can do and what data gets downloaded).
If you're interested, why don't you download your dives into Subsurface? You can then look at the NDLs (or deco requirement) calculated by Subsurface at any point in any dive. You can also see the effect of changing gradient factors or compare Buhlmann gf with VPM-B models. What's reported by your dive computer may also be shown (depending on what your dive computer can do and what data gets downloaded).