Depending on the size of your negatives, the old saw is that you should be able to see details in the shadows, thinnest part of the negative and read news print though the highlight, the darkest part of the negative. If you highlights are blocked to the point you cannot see any detail then you reduce development time. If you prints look flat with muddy highlights you need to increase development time. You can shoot a ring around, I use the method descripted by Carson Graves in his book, The Zone System for 35mm Photographers. In open shade you need a model, fair skin works best, a swath of white cloth, swath of dark cloth, a 18% gray card. For example you are shooting Tmax 400, set your camera or light meter to ISO 25, set the shutter speed and aperture to match the light meter, take a shot. Use your hand or lens cap to over the lens and shoot a blank frame. Then repeat for ISO 50, 100, 200, 400, 800, 1600 and 3200, be sure to keep a blank frame between each shot as it will make it easer to judge later. Develop in your standard developer at the recommended time and temperature. Next using a negative you have gotten an acceptable print as a guide use the same time and aperture with the same paper to print a proof sheet. Once dry see which frame gives you the best shadow detail and highlights from the dark and white cloth and 18% gray card matches your 18% gray card. Once you have a personal E.I or personalized ISO you need not worry about increasing or decreasing development times for a recorded print. At that point you can decide if you to invest time to learn the Zone or Beyond the Zone System.