Source: https://intercom.help/stake-casino/en/articles/4974712-how-to-calculate-your-vip-progress

Please kindly focus on the bold sentence. With that example we can calculate for ryzaadit's case. ryzaadit is 2,3% left to Platinum 3 from Platinum 2, it means he is at 97.7% to Platinum 3. So the calculation should be as follow:
$1M (requirement for platinum 3) - $500k (requirement for Plat 2) = $500k (requirement for him to reach Plat 3 from Plat 2)
97.7% x $500k = $488,500, it means that he need to wager $500k (the requirement) - $488,500 (what he has wagered) = $11,500 to reach Platinum 3.