Unlike JP, we do not have the 11k banner for Charlotte (because we're different games hue); but in its stead, we got a 24k banner with 5% Rainbow rate.

Thus, assuming Gumi doesn't pull another 3.5% Regina thing on us, you can expect the rate to acquire X number of CG Charlotte once you complete the 24k lap ( 10% 5★ rainbow ticket) to be...

Assumptions: 5% Rainbow rate follows the same as Regina's banner, which means from 24k lapis:

• 3★ = 43 units @ 1.5% Banner rate
• 4★ = 10 units @ 5.625% Banner rate
• 5★ Guarantee = 1 unit @ 100% Banner rate
• 5★ at step 3 = 1 unit @ 0.862% Banner rate (1/116 5★ units)
• 10% 5★ ticket from step 4 = 1 unit @ 3.33% Banner rate

Verdict: Understanding that there is Elena coming in probably 2-3 weeks from now, and Galuf, Lenna, Bartz... coming in latest 2 months from now, it may not be advisable to burn lapis on Charlotte; but considering the rate and the unexpected 5% rainbow rate banner, my personal take is that it may be worth it to go all out for 4x Charlotte for that STMR shield, even if the worst case scenario is 12.84% for 4 of her copies.