How to create an alert in a Excel cell based on a condition

I have an account with a brokerage company. When am attempting to sell a limit stock, I get the following error. "The limit price you have entered is too far away from the Bid Price for this security. Please use the following guidelines when entering your limit price: For sell limit orders in which your limit price is above the current Bid Price, Your limit price can be no more than 50% away from the Bid Price. I have a Google sheet with columns with stocks prices etc. Would you have any ideas how to create an alert text as below?

STOCK Symbol: TEST

My Limit Price: $40 (Cell: A1)

Current bid price: $21 (Cell : B1)

In Cell C1: How can I create an alert to appear "fill-limit-order-transaction-now" once the condition to add the limit sell transaction "limit price can be no more than 50% away from the Bid Price" is met.

What I am struggling with is how to come up with a relationship between the target price and the condition..

no more than 50% ....> $21 + 50% of $21...> $31.50... so how can I link this number to my target $40? to come up with an alert condition so I can create a future sell transaction? Thanks!