I wouldn't overcomplicate your nutrition. If you know your BMR and you're able to estimate your TDEE with the amount of walking and physical activity you do on a daily basis, I would simply add 300-400 calories per day above your current maintenance. Using SARM's will help, but your nutrition and planning won't be too much different than if you weren't cycling anything. You can most likely get away with a slightly higher calorie intake, but I would start with a moderate surplus and increase calories as needed.
I would go 300-400 over maintenance for about two weeks and see where you're at. Keep protein high, 1g-1.3g per pound of bodyweight, and do what works best for you with carbs and fats. I usually have an easier time staying leaner if I keep my fat intake relatively low (about 20% of total calories)