how much grain fits in my vessel?

At a ratio of 1 qt water/lb malt, the grain is fully saturated and fills a volume of 42 U.S. fluid ounces. More than that, just add the additional water volume. Once the grain is saturated, more water per pound only adds its own volume.

Hydrometer temperature correction

Table 18 – Hydrometer Temperature Corrections

T °C	Delta G	T °F		T °C	Delta G	T °F
0	-0.0007	32.00		25	0.0021	77.00
1	-0.0008	33.80		26	0.0023	78.80
2	-0.0008	35.60		27	0.0026	80.60
3	-0.0009	37.40		28	0.0029	82.40
4	-0.0009	39.20		29	0.0032	84.20
5	-0.0009	41.00		30	0.0035	86.00
6	-0.0008	42.80		31	0.0038	87.80
7	-0.0008	44.60		32	0.0041	89.60
8	-0.0007	46.40		33	0.0044	91.40
9	-0.0007	48.20		34	0.0047	93.20
10	-0.0006	50.00		35	0.0051	95.00
11	-0.0005	51.80		36	0.0054	96.80
12	-0.0004	53.60		37	0.0058	98.60
13	-0.0003	55.40		38	0.0061	100.40
14	-0.0001	57.20		39	0.0065	102.20
15	0	59.00		40	0.0069	104.00
16	0.0002	60.80		41	0.0073	105.80
17	0.0003	62.60		42	0.0077	107.60
18	0.0005	64.40		43	0.0081	109.40
19	0.0007	66.20		44	0.0085	111.20
20	0.0009	68.00		45	0.0089	113.00
21	0.0011	69.80		46	0.0093	114.80
22	0.0013	71.60		47	0.0097	116.60
23	0.0016	73.40		48	0.0102	118.40
24	0.0018	75.20		49	0.0106	120.20