Not familiar with the Fitzgerald-Spielberger, but it seems more reasonable than the JJ chart which I always felt overvalued high picks.
e.g. the JJ chart says the #1 pick is worth more than 4 TOTAL picks in the 20-25 range while the F-S chart says it's worth 2 total picks.
I'm not sure which chart the draft simulators use, but the JJ chart kinda of explains how you can keep trading down from pick 16 until you can pick the entire 4th round...
The Fitzgerald-Spielberger chart is absolutely insane if used for valuation purposes, and isn't meant for that. It isn't meant to be used as a valuation in terms of the market value of the picks, its a chart to quantify the expected value you get from the player you select.
The FS chart says that the 49th, 50th, and 51st overall are worth more than pick #1 overall.
The number 1 pick IS worth that much, depending on class. Just look at the recent trade for the number 1 pick. The Panthers traded a legit WR1 worth a first himself, 2 first rounders (9 overall and what ended up being #1 this year), and 2 second rounders.
And that was just to trade UP from pick 9. Not to acquire it outright.
FS cannot be used for the purpose of valuing picks for the purpose of trade. The FS chart is just an explicit endorsement for trading down and taking a volume approach to drafting, but it misses a lot of context.