regarding OAT - may I suggest that you get a cheap digital OAT system from your local electronics shop - (KMart and Walmart in the US have them as well). For about $10 you get 0.1°C accuracy which will help tremendously in getting accurate data.
Get one with an 'outside' probe, and then tape the probe outside where it's not influenced by exhaust, etc, and keep the readout in the cockpit.