Specifically - make the planet smaller for gravity.
The formula is F=G(m1+m2/d^2) or
the Force exerted equals the Gravitational Constant ((6.67 x 10 ^-11 N x M^2/kg^2 or .0000000000667 Newtons) multiplied by the mass of the first object + the mass of the 2nd object divided by the distance squared.
If you want the easy way, take object 1 mass, add it to object 2 mass and divide by the distance between the two squared, it isn't perfect, but then you aren't doing a Physics term papaer, you're doing a story.
In your equation the distance is half the radius of the planet. The mass is based upon planet composition (which is why Saturn though larger than Earth has less gravity) and the second object can be any object that you wish to use that you know has a defined mass like a can of peas or a book - do the math and the resulting Force is the actual Gravity of the planet. Keep it and use it as a standard reference for your story.
