Hmmm…. So what is wrong with PO4 being at 0.14? Are the corals telling you they are suffering?
Is the target you are aiming for perhaps the problem and not the actual value you have?
My reef tank was running at values of 0.2-0.5 and all was good.
From my experience if you want to peg your PO4 to some value you need to take measurements before feeding or long after any feeding. Food will swing your PO4 especially coral food “Reef Roids”, frozen foods, phyto etc… With these there is no method that will keep your PO4 flat and you will always be chasing your tail.
Using high quality pellet fish food can help.
Carbon dosing is not instant, it feeds bacteria that in turn consumes NO3 and some PO4. It is biological process that needs time and if the input is unstable the result will be unstable as well.