Question: My total tax liability for the year works out to ₹5,00,000. By 15th September, I was required to have paid 45% of this as advance tax, but I had only paid ₹1,00,000. I paid the shortfall along with the next instalment, 3 months later. Roughly how much interest do I owe on this shortfall?
Answer: Approximately ₹3,750.

Supporting evidence:
title: incometaxindia.gov.in/section-234c-40
quote: the assessee shall be liable to pay simple interest at the rate of one per cent per month for a period of three months on the amount of the shortfall from fifteen per cent or forty-five per cent or seventy-five per cent, as the case may be, of the tax due on the returned income;
context: the advance tax paid by such assessee on its current income on or before the 15th day of June is less than fifteen per cent of the tax due on the returned income or the amount of such advance tax paid on or before the 15th day of September is less than forty-five per cent of the tax due on the returned income or the amount of such advance tax paid on or before the 15th day of December is less than seventy-five per cent of the tax due on the returned income, then, the assessee shall be liable to pay simple interest at the rate of one per cent per month for a period of three months on the amount of the shortfall from fifteen per cent or forty-five per cent or seventy-five per cent, as the case may be, of the tax due on the returned income;
(ii)

the advance tax paid by the assessee on the current income on or before the 15th day of March is less than the tax due on the returned income, then, the assessee shall be liable to pay simple interest at the rate of one per cent on the amount of the shortfall from the tax due on the returned income:
Provided
that if the advance tax paid by the assessee on the current income, on or before the 15th day of June or the 15th day of September, is not less than twelve per cent or, as the case may be, thirty-six per cent of the tax due on the returned income, then, the assessee shall not be liable to pay any interest on the amount of the shortfall on those dates;
(
b
)
an assessee who declares profits and gains in accordance with the provisions of sub-section (1) of
section 44AD
or sub-section (1) of
section 44ADA
, as the case may be, who is liable to pay advance tax under
section 208
has failed to pay such tax or the advance tax paid by the assessee on its current income on or before the 15th day of March is less than the tax due on the returned income, then, the assessee shall be liable to pay simple interest at the rate of one per cent on the amount of the shortfall from the tax due on the returned income:
Provided
that nothing contained in this sub-section shall apply to any shortfall in the payment of the tax due on the returned income where such shortfall is on account of under-estimate or failure to estimate—
(
a
)
the amount of capital gains; or
(
b
)
income of the nature referred to in sub-clause (ix) of clause (
24
) of
section 2
; or
(
c
)
income under the head "Profits and gains of business or profession" in cases where the in