What plastic tab? The one on the engine front beside the crankshaft pulley? If the engine is not running, the mark on the pulley should align with the "0" mark on the indicator.

Are you reading the 15* BTDC with the engine running? If so, that's exactly what the ignition timing should read with a warm engine at ~650 rpm idle. However, the corect way to set the ignition timing is to ground the pin inside the harness connector (located under/near the wiper motor) and set the base timing to 5* BTDC. Then shut off the engine, remove the ground lead, restart the engine and you should read 15* BTDC at the crankshaft timing indicator.

Please clarify exactly how you're going about getting the reading.

Also, when you installed the timing belt, did you align the dot on the crankshaft sprocket with the mark as well as align the marks on the cam sprockets? There are 3 marks that have to be in alignment.

Frank