Hi,
After having been unable to find recommended settings for this combination with my FT-897D, here's what works for me.
Set the WSPR-X slider setting to 25dB and then adjust the Signalink RX pot so that the green bar indicates a receiving level of "around" 30dB.
I'm now regularly getting successful decodes down to -28 or even -30 dB with these settings.
Interestingly after playing around with different settings for a while I also launched the WSPR 2.x software as well and the received level in that window was indicating at around 0dB as suggested!
(Wish I had thought of running the 2 versions together before),
Sorry, should have added laptop soundcard levels at 100%

73 John G8DYK