If you don't have one, install a tempering valve at the WH and set it to 120-degrees or so, and then set the boiler to run the tank to 140 or so, and you should increase the total hot water available. Is the tank setup as a priority zone? That alone might fix the problem. A tank can generally supply somewhere between 70-75% or so of it's volume (without being reheated) before the incoming cold water overwhelms that left in the tank and the temperature starts to drop. I'm assuming this is an indirect tank on a boiler. The valve may have the max temp stop set too low for your winter conditions...pretty much all ant-scald valves have both a pressure balance function and a mechanical stop. If you can move the stop, you may have enough hot water left to keep filling the tub - it depends on how 'cold' it actually gets - can't tell if there's any warm water left in the tank from here and your valve just can't access it without mixing with the cold because the stop is set too low. Hassle with those, is, if you set it in the summer, it may not get hot enough in the winter. If you set it for the winter incoming water temps, it gets too hot in the summer. Course, if you have any sense, you'd not set it high enough, but the regs require this to protect those that might not be able to figure it out on their own.