you can check the flea market to get the current rates! they change pretty frequently as people buy and list, so its best to just go check there :)
also the ratio will be different depending on if you are buying MS or Pebbles (i.e., people who sell MS will sell for a higher pebble:ms rate than those that are selling pebbles) so if you are trading with someone/trying to determine a rate for your sales, its best to take a look at both rates and figure out what you would be comfortable with (usually something in between the two listed prices).
