You want it to be at all times, like a hud message? Or you want to have it just like in your plugin (printed in chat)?
If you want it in chat, like you already have, just in percents, then I guess this should do it:
PHP Code:
public CheckLevel(id)
{
Progress[id] = Exp[id] / ExpNeed[Level[id]] * 100
client_print(id, print_chat, "Exp: %i Progress: %i%% Level: %i", Exp[id], Progress[id], Level[id])
}