Calculate luck in Javascript, based on a RPG stat

challenge.js

/*
	Calculate a RNG value with a percent chance to succeed,
	based on a range and an stat input value somewhere between.
	This is how the challenges in Sunless Sea work, for example.

	- The lower bound defines a starting value required to get over 0%
	- The upper bound defines the value at which you always succeed
	- The stat value should between the two in order to produce a meaningful chance.
*/

let statValue = 138;

let upperBound = 150;
let lowerBound = 40;
let span = upperBound - lowerBound;

let chance = (statValue-lowerBound)/(span/100);
if (chance <= lowerBound) chance = 0;
if (chance >= upperBound) chance = 100;

let randomValue = Math.random();
if (randomValue < chance/100) {
	console.log('You succeeded in a challenge!');
} else {
	console.log('You failed the challenge.');
}

// stats
console.log('The random value was ' + Math.floor(randomValue*100));
console.log('Your stat gave you a chance of ' + Math.floor(chance) + '% to succeed');