To ensure that both the roast and potatoes are done at the same time, you'll want to consider the cooking time for each item. Cooking times can vary depending on the size and type of potatoes, as well as the cut and weight of the roast. However, here are some general guidelines:

1. Roast:
• Preheat your oven to 350°F (175°C).
• For a medium-sized roast (about 3 to 4 pounds), you might need to cook it for approximately 20-25 minutes per pound. However, it's crucial to use a meat thermometer to check the internal temperature. The recommended internal temperature for beef roast is typically around 145°F (63°C) for medium doneness.
2. Potatoes:
• Roasting potatoes at 350°F will usually take about 45 minutes to 1 hour, depending on the size of the potato pieces. You can cut them into smaller pieces for faster cooking.

Here's a suggested plan:

• Place the roast in the oven first since it has a longer cooking time.
• After about 30-45 minutes, place the potatoes in a separate pan in the oven. This way, the potatoes will finish cooking around the same time as the roast.

Remember, these are general guidelines, and it's crucial to use a meat thermometer to check the internal temperature of the roast and the doneness of the potatoes. The internal temperature of the roast is the most reliable way to determine if it's cooked to your desired level of doneness. Adjust the timing based on the actual weights and sizes of your roast and potatoes.