Online calculators can provide a good starting point (e.g., Harris-Benedict, Mifflin-St Jeor formulas, then multiply by an activity factor). Once you have your TDEE, add 300-500 calories for your surplus. Monitor your weight and adjust weekly. If you're gaining ~0.5-1lb per week, you're likely in a good surplus.
What method do you use to estimate your needs?"Knowledge is power, especially when it comes to your metabolism."