A few ways to look at it.
Revenue multiple is the most common: accounts your size usually sell for 2x-4x annual revenue. At $10-12k a year, that puts the account somewhere between $20k-$48k.
Audience value is the other angle. Instagram followers are generally worth $1-3 each depending on engagement and niche. At 39k that's $39k-$117k, but only if engagement holds up. TikTok followers are worth less since loyalty is lower there.
For your buyout specifically, your partner's 11% work contribution doesn't automatically mean 11% of value. If they handle brand relationships or content strategy, that could carry more weight than the hours suggest. Get clear on that before settling on a number.
Check Flippa or Motion Invest for comparable sales. Real transaction data beats any formula.