Percentage change using a base

Hi, I have a panel with items and prices. I have one group of items that belongs to one geographical zone and another group of items that belong to other zone. I would like to compare the inflation rate between this two zones across time but using the same base year (for example when wave == 1). When I want to observe just the variation without a base year I did

Code:

gen price_change_per = (price/price[_n-1] -1)*100 if code==code[_n-1]
egen mean_val = mean(price_change_per), by(wave)

for each of the 2 zones. How should I do if instead of that, I want to to the percentage change always with respect to the same base year? Thanks

ps: as this is my first post in Stata, please let me know if I'm not following the style or the format that should be used for this type of questions.