Dealing with last minute scope changes on a bid day will be a lot easier if you put some thought into your estimate prior to starting your take-off. Using estimating software is essential to making accurate last minute changes! I have blogged about developing your process in the past. If you have developed a well thought out process you can feel confident that you have properly captured the scope changes in your revised estimate.
As estimators become savvier they are able to anticipate potential changes beforehand and setup the estimate in a way which they can easily make changes to it, such as bucketing the estimate the way the project will be built in the field. It will be much easier to locate the effected items in the change if your estimate is bucketed and you have a consistent take-off process!
Extreme caution must be used when incorporating changes at the last moment! It has been my experience that mistakes can easily be made during this period. You are under the gun to make the changes but you can not operate like a run away train out of control. If you catch your nerves taking control of your thought process take a moment and walk away from the project. If you have a co-worker that can lend a hand explain to them what is happening and ask them to review your math and procedure. Don’t forget to acknowledge the change on your scope letter proposal. Take the time to double check your math and go through your process again in your mind because ultimately you will be held accountable if you make a mistake which likely won’t be found until much later. By that time no one will feel your pain when you try and explain what happened in the moments running up to the bid.