The formulae below allowances for the voltage difference due to the wave form factor. The voltage drop across the diodes and the resistance of the choke must be allowed for. We specify the Vac on full load so you do not need to allow for resistance of the transformer windings

dBu	Ratio	Voltage		dBu	Ratio	Voltage
0	1.00	0.7746		0	1.000	0.7746
1	1.12	0.8691		-1	0.891	0.6904
2	1.26	0.9752		-2	0.794	0.6153
3	1.41	1.0941		-3	0.708	0.5484
4	1.58	1.2277		-4	0.631	0.4887
5	1.78	1.3774		-5	0.562	0.4356
6	2.00	1.5455		-6	0.500	0.3882
7	2.24	1.7341		-7	0.447	0.3460
8	2.51	1.9457		-8	0.398	0.3084
9	2.82	2.1831		-9	0.355	0.2748
10	3.16	2.4495		-10	0.316	0.2449
11	3.55	2.7484		-11	0.282	0.2183
12	3.98	3.0837		-12	0.251	0.1946
13	4.47	3.4600		-13	0.224	0.1734
14	5.01	3.8822		-14	0.200	0.1546
15	5.62	4.3559		-15	0.178	0.1377
16	6.31	4.8874		-16	0.159	0.1228
17	7.08	5.4837		-17	0.141	0.1094
18	7.94	6.1528		-18	0.126	0.0975
19	8.91	6.9036		-19	0.112	0.0869
20	10.00	7.7460		-20	0.100	0.0775
21	11.22	8.6911		-21	0.089	0.0690
22	12.59	9.7516		-22	0.079	0.0615
23	14.13	10.9415		-23	0.071	0.0548
24	15.85	12.2765		-24	0.063	0.0489
25	17.78	13.7745		-25	0.056	0.0436
26	19.95	15.4552		-26	0.050	0.0388
27	22.39	17.3411		-27	0.045	0.0346
28	25.12	19.4570		-28	0.040	0.0308
29	28.18	21.8311		-29	0.036	0.0275
30	31.62	24.4949		-30	0.032	0.0245
31	35.48	27.4837		-31	0.028	0.0218
32	39.81	30.8372		-32	0.025	0.0195
33	44.67	34.6000		-33	0.022	0.0173
34	50.12	38.8218		-34	0.020	0.0155
35	56.23	43.5588		-35	0.018	0.0138
36	63.10	48.8737		-36	0.016	0.0123
37	70.79	54.8372		-37	0.014	0.0109
38	79.43	61.5284		-38	0.013	0.0098
39	89.12	69.0360		-39	0.011	0.0087
40	100.00	77.4597		-40	0.010	0.0077

OTHER USEFUL INFORMATION:

To convert max level at 50 Hz to 20 Hz subtract 7.96 dB
To convert an rms voltage to a peak voltage multiply by 1.414
To convert an rms voltage to a p-p voltage multiply by 2.828
To convert a dB value to a ratio in Excel: Ratio =POWER(10,dB/20)
To convert a ratio to a dB value in Excel: dB =20*LOG(ratio)