The flow element drops a certain pressure at a certain maximum flow rate, according to the conditions provided for a calculation.
Any commercial differential pressure flow element (orifice plate, averaging pitot tube, venturi, Taylor wedge, V-cone) comes with a 'sizing sheet' that gives the differential pressure at maximum design flow rate.
Good news: If the DP pressure transmitter has a square root extractor, there's no need to perform a calculation because
0 to max DP (square rooted) = 4 to 20mA = 0 to max flow,
according to the numbers on the sizing sheet.
If the DP pressure transmitter does not have a square root extractor (or it's not used), then the receiver can scale the 4-20mA 0-to-100% (or 0-to-1.00 as shown below) and square root the percentage value, which is the percentage of maximum flow rate.
@ 100% max differential pressure, Sq rt of 1.00 = 1.00 = 100% of max flow rate at design conditions
@ 81% of max pressure; Sq rt of 0.81 = 0.900 = 90% of max flow
@ 50% of max pressure; Sq rt of 0.50 = 0.707 = 70.7% of max flow
@ 25% of max pressure; Sq rt of 0.25 = 0.500 = 50.0% of max flow
@ 20% of max pressure; Sq rt of 0.20 = 0.447 = 44.7% of max flow
@ 15% of max pressure; Sq rt of 0.16 = 0.400 = 40.0% of max flow
@ 10% of max pressure; Sq rt of 0.10 = 0.316 = 31.6% of max flow
@ 5% of max pressure; Sq rt of 0.05 = 0.227 = 22.7% of max flow