Yes, use the 1khz tone.
The Digigal procedure china posted is spot on.
The thing is that with a DR2d line-input, the range of available level adjustment is far more limited than the M10, and totals about 5 dB, which may not be enough to calibrate the meters of both devices so that they read identically.
The input level for the DR2d's line-in needs to be set between 95 and 100. Do not go below 95. If you can get the meters aligned within that region, great. If not you have a few choices:
1) Use sufficient attenuation between the FP24 and DR2d to allow you to get the meters to align.
2) Set the DR2d line-in level to 95 (the least sensitive end of that 95 to 100 range, which should be closest to achieving the correct calibration). Clap or otherwise make noise through the mics while adjusting the gain of the FP24 until the DR2d meters are just about reaching 0dbFS (threshold of clipping) and note that meter reading on the FP24. You may want to mark that point on the FP24 meter. That point is now equivalent to 0dbFS on the DR2d and becomes your never to exceed point on the FP24 meter.
3) Worth a try- Switch to using the mic-input on the DR2d, set to low-gain, and see if you can get closer. Do not go below 65 using mic-input/low-gain. It probably won't but it's worth a shot.