In Sirius you can select a range of reasonable "delta t" values to obtain a range of reasonably accurate house cusps for ancient birth charts. Many of us have become so accustomed to the wonders of science and computer technology that we may not be aware of some limitations, such as the fact that house cusps cannot be calculated with complete accuracy for ancient charts, and therefore house cusps can be incorrect even if the birth data is correct for a chart calculated several hundred years ago or more. For birth charts before about the year 1700 and after about the year 2200 astronomers are unable to precisely determine the exact speed of rotation of the Earth, and consequently house cusps cannot be calculated within 1 minute of arc accuracy.
The astronomical calculation relevant to this issue is called "delta t". In the data entry screen for charts with these data, a "Delta t" button is now available and it provides you the ability to adjust Delta t within a range desired. For example, for a particular year scientists may have determined that delta t is known with 95% accuracy to be within a 1 minute time range. You can adjust delta t to be, for example, at the 95% level above or below the standard value and you can thereby calculate the chart that is less likely to be accurate than with the default setting, but is still possibly correct given the limits of current understanding of the changes in the Earth's rotational speed.