Incorrect Standard Deviation for calculating Sample Size

This is regarding “Video:Calculating Sample Size”. The formula used for calculating the sample size is using the Standard Deviation (SD) of a different sample. Isn’t this incorrect and needs to be validated?
The n comes out to be 267 and the SD used was from a population size of 49. If the new sample of 267 ends up having a large SD then the Margin of Error (MoE) would be higher than 3 cm that we wanted. Should we have a validation step that ensures the SD of the new sample is similar or smaller than the old SD? If it is larger, we might need another larger sample to get the required MoE.

Please give a specific location in the course where this issue is seen.

I have mentioned the video and the week where this calculation is done and explained. Let me know if I can provide any other information.

Please give a time mark within the video.

This is about the concept being covered in the 3 min long video. If a time mark is needed, please use 2:30.