On a Sunday in August we will visit the State Rooms at Buckingham Palace, we already have tickets. Since we are slightly mobility limited, would like a heads up in regards to where the visitor entrance is with already purchased tickets, how much before to appear before 10AM (20 minutes?). If we can save a few steps, so much the better. I will take a taxi from
Covent Garden to get there most likely, unless there is a 'quick' bus from the Seven Dials area. Also how much time will a taxi take from Seven Dials to the best drop off spot.

The entrance is on the south side of the Palace in Buckingham Gate (that is the name of the street)

Normally that would be a 10-15 minute drive depending on traffic. But with the Mall closed to traffic on Sundays, plan on a bit longer since the taxi would have to zig zag around a bit. In your situation, I wouldn't attempt it by bus.

Just tell the cab driver you want the public entrance to the State Rooms in Buckingham gate and he''l put you down right outside the queue. No need to get there much ahead of time 10 or 15 minutes is plenty. As was explained on your other threads - the tickets are timed and everyone in your time slot will be admitted at the same time. Front of the queue or back of the queue really doesn't matter much.

If you read the website you can see where you enter, with directions and information about timings, clothing and so on. Map and directions under one tab, practical information under another.
https://www.rct.uk/visit/the-state-r...kingham-palace