I will have a play and get back to you - obviously not the best method then ! But in my defence I was more interested in getting the formatting right, but still no good as it stands - "I'll be back..."
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Back Give this a try - it will still not be 100%.
If the updated capx (previous posted one updated) if still no good let me know and I will try with wallclock...
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I added a silly loop to randomly rotate quite a few largish sprite to deliberately slow down my computer (from 60 fps to about 28 fps). with this loop running a five minute countdown took 302 seconds without the loop it took 300.043 seconds. Obviously using every tick take the dt from a counter allows for a slight error - this error is greater the higher the dt.