Calculate price that is 90% greater than previous close

Since you have requested a custom watchlist column I have moved your post out of the "Chart Studies" topic and into the "Watch Lists" topic.

The simplest way to compute this is to set the custom column to the daily time frame. In this way we only need to reference the previous bar. So this code will compute the value that is 90% above the close of the previous bar (as apposed to only the previous day), regardless what time frame you select for the custom column.

input percentChange = 90.0;
plot data = close[1] * (1 + percentChange * 0.01);

I have included a user input to adjust the percent change each user might want to display.