Very wrong. You need to calculate the air flow through the engine. Nothing to do with thrust.
An example. Which I based upon figures from the Canadian VAAC, which seems to use a distinctly better model than the British one, with results divided in low, medium and high concentrations. Low goes from 10 to 100 micrograms per cubic meter of air. If you consider a plane flying at 180 m/s, an engine with a capture area of 3 square m, and a concentration corresponding to the upper limit of the "low" concentration, over ten minutes (an estimate of the time to cross a lower level cloud, after takeoff or before landing), you absorb roughly 0.3 grams of ash per engine.
Ok.
So (180m/s x 3m) *10microgram=5400microgram/second.
5400microgram * 10*60= 3.24grams in 10 minutes.
At a higher concentration (still low) of 100 micrograms per cubic meter of air it would be 32.4g per engine per transit through the layer.
This is assuming 10 minutes transit, I believe the cloud is now down to FL10 and below isn't it?