Didn't have any dc coils to test until just a few days ago, but here's some real world data that may be useful for comparison purposes. I tested the field strength on both ends of a 24V, 8W solenoid, using a Honeywell SS495A and an Arduino.
On the more open, exposed end, I found:
500+ Gauss touching the end
100 Gauss @ 1/4"
40 Gauss @ 1/2"
On the business end, which was pulling a small plate tight against itself, I measured:
103 Gauss touching the plate
54 Gauss @ 1/4"
28 Gauss @ 1/2"
Depending on what size relays you're testing, their coils may be weaker or stronger than this.
