Welcome to the site !!
Not sure on the amount of coolant lost , but when trail riding these bikes (i have a 02) do spit coolant as the radiators are smaller than the X and they just don't do well lugging on trails without a overflow tank , but i trail ride mine all the time , and in doing so i have mostly solved this issue , here is what i suggest
Get a 1.6 or 1.8 Bar Radiator Cap , the stock 1.2 will allow coolant to burp out even at the slightest sign of heat , the higher pressure cap will allow the pressure to build higher before letting coolant out , so it will still spit fluid , but not as easily
Next , go on eBay and find a 250X overflow bottle , you can mount it on the left rear by making a simple bracket and attach it to the seat and fender bolt , this will allow the majority of the coolant to go into the overflow tank , then when the bike cools , it returns to the radiator , you will still lose a slight amount from any steam that escapes , but you will never get 100% containment , so that is normal , even for the 250X and 450X
Next is switch from coolant to either Engine Ice , or Evans Waterless Coolant , (read the directions carefully on flushing the system) , either of these will allow up to 50 degrees more temperature before a boil over , which along with the other recommendations i mentioned , should make you happy and able to ride the toughest of single track without worrying about boiling over and getting stuck out in the woods unless your sitting there idling for 5 minutes , then nothing will stop that unless you can add a cooling fan and a battery to run it
Hopefully this gives you some ideas and a place to start....
Here is my setup: