I do believe the easiest way of finding TDC; Before removing belt!, is find TDC crank mark on flywheel, check for tdc at injection pump, check cam tdc(#1cyl). now you can lock pump with pin.
there are multiple tdc(s). four in each stroke; here we're interested in the combustion/compression stroke, and TDC, im wording it 'mechanical TDC', as cylinder #1 TDC is what we're shooting for. mark on flywheel gives you cyl#1 TDC, both cam lobes pointing 'up' on #1 is cam tdc, and pump locking with pin is injection pump 'tdc'apx. the 'up' (better described in earlier posts) of the camshaft lobes shows you your coming off compression stroke to injection(and combustion)(combustion is increasing/at end on compression cycle/stroke).
if the belt happens to be removed(for whatever reason) I believe the best way to get to TDC is to remove camshaft(Correctly), with cam removed find crank/flywheel tdc and pump tdc. you should consult Bentley for proper way to go from here, I'd just install cam at #1tdc and be fine, but the manual may include more, and the proper way to remove/install camshaft.
the TDC marks I have seen are all stock, and all my marks are on(not including wear). if your marks don't line up correctly I cannt help, but with 1 small timing hole(or the correct pump hole for pin) and lock pump with pin, and go from there. this is for standard/stock TDC marks.