bluefalcon,
I'm definitely not qualified to argue the toss on IAS/EAS/RAS/CAS/TAS etc, but my guess is that your 5kts per 1000ft is for 250kts indicated. That's 2% per 1000ft, which is probably closer to the truth than my 1.5%, but will lead to a serious overestimate for 140ks IAS.
I'm just conjuring up the image of a hangglider leaping off a 10,000ft cliff and hitting 50kts in a nanosecond
Anyway, as I say, I'm not qualified & ain't going to argue!