If you're really asking if you can calculate it reliably so you can avoid just measuring it, then the answer to that is "no".
Here are some of the key components to the latency difference calculation:
A. Your DSL gateway's latency from DSL to Ethernet may be different from it's latency from DSL to Wi-Fi. How much that is, is impossible for us to know, because it depends on the device's internal implementation.
B. Minimum latency of a single Ethernet link is reasonably calculable if you know what flavor of Ethernet you're running: 10BASE-T, 100BASE-TX, 1000BASE-T. But without knowing the Ethernet flavor, we can't calculate that.
C. Your Netgear router has a processing latency between Ethernet and Wi-Fi, but again, it's implementation dependent and we can't calculate it.
D. There may be an additional latency difference if your Netgear router uses a different flavor of Wi-Fi than your DSL gateway does.
So A and C are basically incalculable, and B and D might be somewhat calculable if we had more information, and the total latency change is A+B+C+D.
Overall it's easier if you just measure the latency increase yourself. It doesn't take too long to run some pings the way you have it right now, then hook things up the other way and run some more pings.