You can pretty much ignore the stock rate they are reporting. Because the stock springs are progressive, the rate they provide is not a good indicator of how the straight rate springs will compare. Basically, the stock springs start at a rate much lower than 0.98 and then increase to somewhere around 0.98 as the spring fully compresses.
You'll actually find it easier to set correct sag with a proper straight-rate spring.
In your case, 0.90 would probably be the way to go.